Digital radiography provides a two-dimensional (2-D) image of a three-dimensional (3-D) object resulting in superposition of structures. Stereoscopic imaging is a technique wherein at least two 2-D x-ray projection images, referred to as an image pair, separated by an angle not exceeding 20-degrees (typically, 3 to 10 degrees) are acquired and displayed on a stereo-capable display. Such display may include two monitors each displaying one of the projection images, and the displayed projection images are viewed through cross-polarized mirrors and lenses, resulting in one eye visualizing one image and the other eye the other image. Alternatively, the images can be visualized using “3-D displays” such as those used in consumer electronics. In typical implementation of stereoscopic imaging, the image pair is acquired by physical movement of a single x-ray tube. Digital tomosynthesis is a technique wherein a plurality of 2-D x-ray projections are acquired over a limited angular range not exceeding 180-degrees (typically, 15 to 90 degrees) and mathematically reconstructed to provide a quasi-tomographic or 3-D image of object. This technique has the potential to improve detection of an abnormality in body anatomy and is being actively investigated for breast, chest and abdominal imaging. FDA-approved clinical systems for chest imaging and for breast imaging have been developed by various manufacturers.
In typical implementation of digital tomosynthesis, (n+1) projections are acquired over an angular range of −θ to +θ spanning 2θ degrees. Here n is a positive integer. Typically, the peak tube voltage (given in kilovolts peak, or kVp) applied across the anode-cathode of the x-ray tube, and the anode target and x-ray beam filter, referred to target/filter are maintained the same during acquisition of the (n+1) projections. The kVp and target/filter define the x-ray spectral shape, and in combination with the tube current (in millamps), or mAs where mAs is the product of tube current in mA and the x-ray exposure duration in seconds) define the x-ray fluence (photons per unit area). In early studies (see Niklason, L. T., B. T. Christian, L. E. Niklason, D. B. Kopans, D. E. Castleberry, B. H. OpsahlOng, C. E. Landberg, P. J. Slanetz, A. A. Giardino, R. Moore, D. Albagli, M. C. DeJule, P. F. Fitzgerald, D. F. Fobare, B. W. Giambattista, R. F. Kwasnick, J. Q. Liu, S. J. Lubowski, G. E. Possin, J. F. Richotte, C. Y. Wei, and R. F. Wirth, Digital Tomosynthesis in breast imaging. Radiology, 1997. 205(2): p. 399-406; Suryanarayanan, S., A. Karellas, S. Vedantham, S. P. Baker, S. J. Glick, C. J. D'Orsi, and R. L. Webber, Evaluation of linear and nonlinear tomosynthetic reconstruction methods in digital mammography. Acad Radiol, 2001. 8(3): p. 219-24; and Suryanarayanan, S., A. Karellas, S. Vedantham, S. J. Glick, C. J. D'Orsi, S. P. Baker, and R. L. Webber, Comparison of tomosynthesis methods used with digital mammography. Acad Radiol, 2000. 7(12): p. 1085-97), tube current was also maintained the same during acquisition of the (n+1) projections.
In current practice, the multiple views required for tomosynthesis require the physical rotation of the x-ray tube for each tomographic view. Although this is technically attainable, the physical movement of the tube is the source of many problems in tomosynthesis. A moving x-ray tube prolongs the exposure time and the duration of physical compression of the breast that in turn increases patient discomfort. Moreover, the resulting longer image acquisition time is more likely to contribute to blurring of the images due to patient motion and physical movement of the x-ray tube.
Systems using multiple stationary x-ray sources have been described by others for use in tomosynthesis, particularly for breast imaging (Kautzer et al. US 2005/0226371 A1 and U.S. Pat. No. 7,330,529 B2, Ludwig et al. US 2010/0091940 A1, Zhou et al. U.S. Pat. No. 7,751,528). However, such systems lack a central high power x-ray tube or other type of high-power x-ray source. A tomosynthesis system with a series of stationary sources will have no capability of performing conventional digital mammography because this requires a relatively high power x-ray source to provide sufficient x-ray fluence rate (defined as the number of x-ray photons per unit area per unit time, or x-ray fluence per unit time) that meet mammographic requirements for a reasonably short x-ray exposure, typically between 0.3 to 2.0 second duration. The fixed multi-spot x-ray sources are significantly underpowered and currently they are not capable of delivering the high x-ray fluence needed for mammography in an acceptable time frame to minimize patient motion. Therefore, such systems will be limited to tomosynthesis use only and not mammography. However, a mammographic or radiographic unit that can only operate in the tomosynthesis mode is too limited and it will not be desirable in most medical practices, because most breast imaging centers would prefer a system that can perform both tests. The same reasoning applies to digital radiography and tomosynthesis of other parts of the body such as chest, abdominal and pelvic imaging.
Some mammographic and radiographic imaging systems currently manufactured can perform conventional digital mammography and tomosynthesis on demand. In the conventional approach, tomosynthesis can be performed by a mechanical scan of the rotating anode x-ray tube over an arc of about +/−30 degrees from the center and acquiring typically from 15 to 25 images across this scan. The detector may remain stationary or it can rotate and/or move laterally to track the x-ray beam. Each of the 15 to 25 images require a combination of rapid activation of the x-ray tube (termed as “fire”) followed by a mechanical movement to the next position for the next fire or x-ray source activation. This rapid firing and mechanical repositioning creates many problems that lead to a less than optimal tomosynthesis image acquisition. During each firing there is a rise and fall “pulse” of the x-ray tube voltage. The tube current and x-ray output can be hard to control without elaborate and expensive electronic controls. Any irregularities in this pulse can contribute to an increased dose to the patient particularly due to the slow rise and drop of the waveform. Moreover, the x-ray filament is susceptible to the mechanical vibrations of the movement of the tube and this can have a negative effect on the spatial resolution of images. A mechanical “stop-fire-and-go” approach is very problematic because of the mechanical instabilities due to acceleration and deceleration of the mechanical assembly of the x-ray source. Continuous mechanical movement is generally preferred but it also prone to vibrations that can affect the image quality. In addition, during each firing that is of finite duration, the x-ray tube is in continuous motion resulting in blurring that degrades image quality. It typically takes between four to ten seconds for a complete acquisition and this increases the chances for a slight movement of the breast or other part of the body that will degrade the spatial resolution and diagnostic quality of the images. This motion problem can be minimized by applying additional compression on the breast using the pneumatic compression mechanism and plate but this is highly undesirable because of increased pain or discomfort. In chest and abdominal radiography this problem is even more serious because shorter exposures are required, typically in the order of milliseconds in chest imaging. A published international patent application by Ren et al. (WO 2010/060007 A1) discusses some of the issues of mechanical scanning approaches.
Other prior art known to the inventors includes the following patents and published applications.
U.S. Pat. No. 6,649,914 issued to Moorman et al. on Nov. 18, 2003 is said to describe an x-ray imaging system according to the present invention comprising a stepped scanning-beam x-ray source and a multi-detector array.
U.S. Pat. No. 7,099,435 issued to Heumann on Aug. 29, 2006 is said to describe a tomographic reconstruction method and system incorporating Bayesian estimation techniques to inspect and classify regions of imaged objects, especially objects of the type typically found in linear, areal, or 3-dimensional arrays.
U.S. Pat. No. 7,545,907 issued to Stewart on Jun. 9, 2009 is said to describe a method of obtaining projection data of an object from a plurality of view angles with respect to the object is provided. The method comprises acts of providing radiation, at each of the plurality of view angles, to an exposure area in which the object is positioned, controlling a radiation energy of the radiation provided at each of the plurality of view angles such that the respective radiation energy is different for at least two of the plurality of view angles, and detecting at least some of the radiation passing through the exposure area at each of the plurality of view angles to obtain the projection data.
U.S. Pat. No. 7,551,716 issued to Ruhrnschopf on Jun. 23, 2009 is said to describe scatter correction methods for breast imaging and is relevant to the “scatter compensation in tomosynthesis” aspect of our disclosure. The approach described by Ruhrnschopf uses a pre-computed library of scatter spread functions using Monte Carlo simulations, which is a standard computational tool for scatter estimation. We have published previously Monte Carlo simulations of scatter as a function of tomosynthesis projection angle. See Sechopoulos, I., S. Suryanarayanan, S. Vedantham, C. J. D'Orsi, and A. Karellas, Scatter radiation in digital tomosynthesis of the breast. Med Phys, 2007. 34(2): p. 564-76.
U.S. Patent Application Publication No. 20090268865 A1 (Ren et al.) published on Oct. 29, 2009. This patent application is said to describe a method and an apparatus for estimating a geometric thickness of a breast in mammography/tomosynthesis or in other x-ray procedures, by imaging markers that are in the path of x-rays passing through the imaged object.
U.S. Pat. No. 7,616,801 issued to Gkanatsios et al. on Nov. 10, 2009 is said to describe a method and system for acquiring, processing, storing, and displaying x-ray mammograms Mp and tomosynthesis images Tr representative of breast slices, and x-ray tomosynthesis projection images Tp taken at different angles to a breast, where the Tr images are reconstructed from Tp images.
U.S. Patent Application Publication No. 20100063410 A1 (Avila), published Mar. 11, 2010, describes obtaining a lung cancer risk index based on combining information from multiple sources such as spirometry, chest CT or other x-ray examination including x-ray tomosynthesis with airflow lung function measurements.
U.S. Pat. No. 7,680,240 issued to Manjeshwar on Mar. 16, 2010 is said to describe methods for performing image reconstruction that include deriving background projection data for an area outside a targeted field of view of a tomographic image, and reconstructing the tomographic image of the targeted field of view, wherein the background projection data is used in the reconstruction.
U.S. Pat. No. 7,697,661 issued to Souchay et al. on Apr. 13, 2010 is said to describe a method wherein the irradiation dose to the breast is distributed in a manner based on the orientation of the x-ray beam followed by filtering of the projections to ensure optimum propagation of the signal-to-noise ratio.
U.S. Pat. No. 7,702,142 issued to Ren on Apr. 20, 2010 is said to describe a method and a system for using tomosynthesis projection images of a patient's breast to reconstruct slice tomosynthesis images such that anatomical structures that appear superimposed in a mammogram are at conforming locations in the reconstructed images.
U.S. Pat. No. 7,751,528 issued to Zhou on Jul. 6, 2010 is said to describe using a stationary array of x-ray sources to facilitate tomosynthesis.
Other references known to the inventors include the following non-patent literature: Nishikawa, R. M., I. Reiser, P. Seifi, and C. J. Vyborny, A new approach to digital breast tomosynthesis for breast cancer screening, in Medical Imaging 2007: Physics of Medical Imaging, J. Hsieh and M. J. Flynn, Editors. 2007, SPIE. p. 65103C; and Sechopoulos, I., S. Suryanarayanan, S. Vedantham, C. D'Orsi, and A. Karellas, Computation of the glandular radiation dose in digital tomosynthesis of the breast. Med Phys, 2007. 34(1): p. 221-32.
A number of problems in attempting to implement a method and system that provides both radiographic and tomographic images have been observed.
There is a need for methods and systems that provide radiographic, stereoscopic and tomographic images.